The present invention relates to a method and apparatus for measuring a gap between adjoining fuel rods disposed in a fuel assembly and a gap between an adjoining fuel rod and water rod disposed therein; and more particularly to the method and apparatus wherein fluid, such as cooling water or specific gas, distribution can be precisely obtained for a short time from a tomographied image of the fuel assembly utilizing a nuclear magnetic resonance phenomenon to thereby measure the gap between the adjoining fuel rods as an image These gaps will be inclusively referred to herein as a gap between the fuel rods as far as no problem is raised.
Usually, a nuclear reactor has a reactor core into which a plurality of fuel assemblies are charged, and a typical one example of one of such fuel assembly is shown in FIG. 33, which represents a fuel assembly to be charged into a reactor of a boiling water type nuclear reactor.
Referring to FIG. 33, a fuel assembly 1 comprises a square cylindrical channel box 2 in which a plurality of fuel rods 3 and at least one water rod are supported in lattice in cross section with spaces with each other by spacers 4. Both upper and lower end portions of each of the fuel rods 3 are secured in an assembled state to upper and lower tie plates 5 and 6. The gaps between the adjoining fuel rods 3 and between the adjoining fuel rod 3 and water rod are supported by the spacers 4, and in an arrangement of the fuel assembly 1 in which the fuel rods 3 of 8 rows and 8 column lattice are charged, each gap is small, 4 mm for example.
It is significant to form gaps between the fuel rods 3 for transferring heat from the surfaces of the fuel rods to the coolant. In a case where the gap is below a certain predetermined value, this heat transfer phenomenon transits from a nucleate boiling phenomenon having good heat transferring property to a film boiling phenomenon having bad heat transferring property, which may result in an excessive increasing of a surface temperature of a fuel cladding. Because of such reason, inspection of the gaps between the fuel rods in the fuel assembly is performed to ensure the proper gap is maintained at a time of periodical inspection generally carried out after about one year running of a nuclear power plant or at a time of manufacturing the fuel assembly.
Namely, the nuclear power plant is generally subjected to the periodical inspection for about three months after about one year operation, and in such periodical inspection, respective equipments are inspected and nuclear fuel is exchanged with new fuel. For instance, with a boiling water type reactor, including 764 fuel assemblies, having output power of about 1.1 milion kw, about one fourth of the fuel assemblies is exchanged with new ones and the remaining three fourths thereof is continuously used. The fuel assemblies to be exchanged are herein called spent fuel and the remaining fuel assembles which are not changed are called re-charged fuel.
Sampling inspection is performed by sampling predetermined number of spent fuels and the re-charged fuels in accordance with every design type of the fuel assemblies. The sampling inspection includes an outer appearance inspection of the fuel assemblies and an inspection of a gap between the adjoining fuel rods. The gap measuring method in such sampling inspection will be described hereunder with reference to FIG. 34.
FIG. 34 shows a brief arrangement in cross section of a fuel assembly 1, taken along the line XXXIV--XXXIV in FIG. 33, including 62 fuel rods 3 and two water rods 7 arranged centrally in the arrangement forming a lattice of 8 rows and 8 columns.
In the prior art, a method of measuring a gap between the adjoining fuel rods is performed by positioning a light source to an A side, for example, observing the light passing the gaps between the fuel rods 3 on a C side opposing to the A side as shown in FIG. 34 by means of a submerged camera, for example, and observing a light generated from a light source positioned to a B side and passing the gaps between the fuel rods 3 on a D side opposing to the B side also by means of the submerged camera. These lights are therefore projected in crossing directions to each other with the right angle. This method for observing the gaps between fuel rods is called herein a light projection method for the sake of convenience.
In a case where it is observed by the light projection method that the gaps between the fuel rods become considerably narrow, a feeler gauge of a plate gauge structure having known thickness is inserted into the gap to observe the condition thereof. This method is called a feeler gauge method.
In these days, it is required to make a high degree of burnup of the fuel assembly for the purpose of improving fuel economy and hence to develop a new type fuel. In order to make high the degree of burnup, it will be necessary to sample data of neutron irradiation character in the fuel assembly and to reflect the data to the design of the fuel assembly. Although it is desired that the data includes sampling data regarding the gap between the fuel rods of the fuel assembly, the above mentioned light projection method and feeler gauge method are not sufficient. That is, in these methods, a lot of feeler gauges are required in order to obtain a required accuracy of the measured gap data, and for example, when the accuracy of order of 0.05 mm is required, 50 plate gauges will be required. Moreover, in the case of measuring the spent fuel assembly, a remote control operation with a distance about 5 m in submerged condition will be required because the fuel assembly is a source of high radiation. For this and other reasons, even if the required feeler gauges are prepared, much attention will have to be paid to the insertion angle of the gauges into the gaps between the fuel rods and the inserting force in order to achieve the operation with high accuracy, thus being inconvenient and troublesome.
In a future nuclear reactor, a fuel assembly having an arrangement aimed at the achievement of high degree of burnup and FIG. 35 shows a brief arrangement of such fuel assembly. The assembly 1a of FIG. 35 is shown in cross section of a lattice of eight rows and eight columns including one water rod disposed centrally in the arrangement of the fuel assembly 1a. The water rod 7a occupies a space corresponding to a space of four fuel rods 3a, hence being called a large diameter water rod 7a. This cross section is taken along the line corresponding to a portion of FIG. 34.
In a case where it is required to observe the gaps between the fuel rods 3a of the fuel assembly 1a of FIG. 35, three gaps at the central portion cannot be observed by the conventional light projection method because of the location of the large diameter water rod 7a. Moreover, when the feeler gauges are inserted into the gaps, it is difficult to precisely measure the gaps G between the fuel rods 3a and the centrally arranged large water rod 7a. The measurement of the gaps of this fuel assembly 1a by the conventional method takes much time and labor.
Recently, to eliminate the defects or drawbacks encountered in the above prior art, there has been studied and provided a method of measuring radial positions of the fuel rods and water rods by utilizing a high energy X-ray CT (cathod tube) or high engergy .gamma.-ray CT for obtaining a tomographied image of an object. However, this method also involves a problem such that since this method utilizing the radiation is carried out in a short time because of the high radiation souce of the spent fuel assembly in addition to the usage of the radiation and the location of uranium dioxide UO.sub.2 having high shielding ratio, it is not suitable for obtaining a significant S/N ratio in a short of measuring. Accordingly, in order to obtain a high precision measuring result, a long measuring time is required, thus being not practical. Moreover, in the case of the high engergy .gamma.-ray CT, treatment or handling thereof itself involves a troublesome problem.